Television cameras are in widespread use in a variety of sensing, communication, and scientific applications. With the advent of improvements such as high-definition systems, camera defects will become more apparent to the television viewer. Thus, camera improvements are desirable. One of the defects to which cameras are subject is "dark current" or "black shading", which is current or charge (signal) which arises from the characteristics of the camera imager (the actual photosensitive screen) itself, and which does not depend on the image falling thereon.
The dark signal may be viewed as being the imager signal when a cap is placed on the associated lens to eliminate light from the image. In the context of charge-coupled device (CCD) imagers, the dark current may be viewed as "leakage" which results in charge which accumulates in each picture element (pixel) of the "A" register of the imager during the image integrating interval. Thus, each pixel includes a charge portion attributable to the integration of image information during the integrating interval, and also includes a charge portion attributable to dark current over the same interval.
Black shading errors, i.e., non-uniformities in the capped-black image of a video camera, have been corrected in various ways. The most basic method consists of the user adding varying amounts of DC, sawtooth and parabolic waveforms to the video signal in an attempt to remove these non-uniformities. This has the advantage of requiring very little memory since only the amplitude coefficients of the simple waveforms are stored. While this may have worked reasonably well with camera tubes, it does not lend itself to CCD correction due to the discrete nature of the sensing elements and the large number of high order waveforms needed for adequate correction.
Another technique which works with CCD cameras is to store the entire capped-black field in a large memory and subtract this from the video signal during operation to remove shading errors. While this results in pixel by pixel correction, it is very expensive and hardware intensive since a full field of memory is required for each channel.